Transducer

Abstract

A passive electrically operable monitoring means has an inherent resistance and a characteristic capacitance and inductance only, so that the monitoring means has a predetermined natural frequency of oscillation. Variations in a physical parameter being monitored by the monitoring means will cause the characteristic capacitance and/or inductance to vary thereby varying the natural frequency of oscillation of the monitoring means. A transducer includes an excitation means for exciting the monitoring means and a sensing means for sensing the natural frequency of oscillation of the monitoring means. The excitation means includes a pulse generating means for exciting the monitoring means with a string of pulses having a repetition rate suitably lower than the natural frequency of oscillation of the monitoring means so that damped oscillations occur in the monitoring means.

We claim: 
     
       1. A passive electrically operable monitoring means for use with a transducer for monitoring a physical parameter, the monitoring means having an inherent resistance and a characteristic capacitance and inductance only, the characteristic capacitance and inductance being responsive to variations in the physical parameter such that the monitoring means has a natural frequency of oscillation that varies in accordance with variations in the physical parameter. 
     
     
       2. The monitoring means as claimed in claim 1, which includes a length of conducting wire wound into a coil with spaces being provided between adjacent turns of the coil. 
     
     
       3. The monitoring means as claimed in claim 2 in which the turns are resiliently arranged relative to one another. 
     
     
       4. The monitoring means as claimed in claim 3 in which the physical parameter being monitored is force and which includes a resiliently flexible web arranged between the turns of the coil, with the turns being secured to the web so that, in use, as the web flexes under the effect of the force being monitored, the spacing between the turns varies, thereby varying the characteristic inductance and capacitance of the coil, and hence its natural frequency of oscillation. 
     
     
       5. The monitoring means as claimed in claim 2 which includes a conduit defining means comprising a length of tube which is coiled with the length of wire of the coil so that the tube is arranged between the turns of the coil. 
     
     
       6. The monitoring means as claimed in claim 2 in which the physical parameter being monitored is force and which includes a dielectric medium located between the turns of the coil, the permittivity of the dielectric medium being variable in response to variations in the force applied thereto, for varying the natural frequency of oscillation. 
     
     
       7. The monitoring means as claimed in claim 1 which comprises at least two lengths of spaced conductors arranged in parallel spaced relationship relative to one another. 
     
     
       8. The monitoring means as claimed in claim 7 in which the physical parameter being monitored is force and in which the spacing between the conductors is variable to vary the natural frequency of oscillation of the monitoring means. 
     
     
       9. The monitoring means as claimed in claim 7 in which the physical parameter being monitored is temperature and which includes a conduit defining means for defining a conduit whereby a gas can be introduced into spaces in proximity to the conductors so that as the temperature of the gas varies the permittivity of the gas between the conductors varies thereby varying the natural frequency of oscillation. 
     
     
       10. The monitoring means as claimed in claim 7 in which the physical parameter being monitored is force and which includes a dielectric medium located between the lengths of conductors, the permittivity of the dielectric medium being variable in response to variations in the force applied thereto, for varying the natural frequency of oscillation. 
     
     
       11. A transducer for monitoring a physical parameter, the transducer including a monitoring means as claimed in claim 1;   an energising means for energising the monitoring means; and   a sensing means for sensing the natural frequency of oscillation of the monitoring means.   
     
     
       12. The transducer as claimed in claim 11 in which the energising means includes an energising coil which is coupled electrically and/or magnetically to the monitoring means in a wireless manner for energising the monitoring means to cause the monitoring means to oscillate at its natural frequency of oscillation. 
     
     
       13. The transducer as claimed in claim 12 which includes a pulse generating means for generating a string of pulses at a repetition rate sufficiently lower than the natural frequency of oscillation of the monitoring means so that damped oscillations occur in the monitoring means, in use. 
     
     
       14. The transducer as claimed in claim 11, in which the sensing means includes a sensing coil which is electrically and/or magnetically coupled to the monitoring means in a wireless manner. 
     
     
       15. A tire which includes a casing; and   a monitoring means as claimed in claim 1 inclusive mounted within and rotatably fast with the casing.   
     
     
       16. A method of monitoring a physical parameter which includes energising a passive electrical monitoring means having only a characteristic capacitance and inductance and an inherent resistance so that the monitoring means oscillates at a natural frequency of oscillation;   causing the characteristic capacitance and the characteristic inductance to vary in response to variations in the parameter being monitored thereby to vary the natural frequency of oscillation of the monitoring means; and   sensing the variations in the natural frequency of oscillation of the monitoring means to provide an indication of the value of the parameter being monitored.   
     
     
       17. The method as claimed in claim 16, in which the parameter being monitored is force, the monitoring means comprises a length of conducting wire wound into a coil to provide a plurality of spaced turns, and in which the method includes varying the spacing between the coils to vary the characteristic capacitance and inductance of the coil, and hence its natural frequency of oscillation, in response to variations in the force being applied to the coil. 
     
     
       18. The method as claimed in claim 16 in which the monitoring means comprises at least two lengths of spaced conductors which are arranged in parallel spaced relationship relative to one another, and in which, when the parameter being monitored is force, the method includes varying the spacing between the conductors. 
     
     
       19. The method as claimed in claim 16 in which the monitoring means comprises at least two lengths of spaced conductors which are arranged in parallel spaced relationship relative to one another, and in which, when the parameter being monitored is temperature, the method includes introducing a gas into a space in proximity to the conductors so that as the temperature of the gas varies, the permittivity of the gas between the conductors varies thereby varying the natural frequency of oscillation. 
     
     
       20. The method as claimed in claim 16 in which the parameter being monitored is force, and monitoring means comprises a length of conducting wire wound into a coil to provide a plurality of spaced turns, and in which the method includes varying the permittivity of a medium between the turns in response to variation in the said force thereby to vary the natural frequency of oscillation of the coil. 
     
     
       21. The method as claimed in claim 20 which includes locating a suitable material between the turns, the material being allowed to expand or to be compressed as the force being monitored, and which is exerted on the material, varies so that the permittivity of the material varies. 
     
     
       22. The method as claimed in claim 21 which includes introducing a suitable material between the elements in proportion to the force being monitored, sot hat the permittivity of the medium between the turns varies. 
     
     
       23. The method as claimed in claim 16 which includes energising the monitoring means with a string of pulses. 
     
     
       24. The method as claimed in claim 23 which includes energising the monitoring means with a string of pulses which has a repetition rate suitably lower than the natural frequency of oscillation of the monitoring means so that damped oscillations occur in the monitoring means. 
     
     
       25. The method of monitoring as claimed in claim 16 in which the physical parameter being monitored is humidity and which includes a conduit defining means for defining a conduit whereby a gas, the humidity of which is to be monitored, can be introduced into spaces in proximity to the conductors so that as the humidity of the gas varies the permittivity of the gas between the conductors varies thereby varying the natural frequency of oscillation. 
     
     
       26. The method as claimed in claim 16 in which the monitoring means comprises at least two lengths of spaced conductors which are arranged in parallel spaced relationship relative to one another and in which, when the parameter being monitored is humidity, the method includes introducing a gas into a space in proximity to the conductors so that as the humidity of the gas varies, the permittivity of the gas between the conductors varies thereby varying the natural frequency of oscillation.